/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.tomcat.util.http.fileupload;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * An output stream which will retain data in memory until a specified
 * threshold is reached, and only then commit it to disk. If the stream is
 * closed before the threshold is reached, the data will not be written to
 * disk at all.
 * <p>
 * This class originated in FileUpload processing. In this use case, you do
 * not know in advance the size of the file being uploaded. If the file is small
 * you want to store it in memory (for speed), but if the file is large you want
 * to store it to file (to avoid memory issues).
 */
public class DeferredFileOutputStream
		extends ThresholdingOutputStream {

	// ----------------------------------------------------------- Data members

	/**
	 * The temporary file prefix.
	 */
	private final String prefix;
	/**
	 * The temporary file suffix.
	 */
	private final String suffix;
	/**
	 * The directory to use for temporary files.
	 */
	private final File directory;
	/**
	 * The output stream to which data will be written prior to the theshold
	 * being reached.
	 */
	private ByteArrayOutputStream memoryOutputStream;
	/**
	 * The output stream to which data will be written at any given time. This
	 * will always be one of <code>memoryOutputStream</code> or
	 * <code>diskOutputStream</code>.
	 */
	private OutputStream currentOutputStream;
	/**
	 * The file to which output will be directed if the threshold is exceeded.
	 */
	private File outputFile;

	// ----------------------------------------------------------- Constructors

	/**
	 * Constructs an instance of this class which will trigger an event at the
	 * specified threshold, and save data to a file beyond that point.
	 *
	 * @param threshold  The number of bytes at which to trigger an event.
	 * @param outputFile The file to which data is saved beyond the threshold.
	 */
	public DeferredFileOutputStream(int threshold, File outputFile) {
		this(threshold, outputFile, null, null, null);
	}

	/**
	 * Constructs an instance of this class which will trigger an event at the
	 * specified threshold, and save data either to a file beyond that point.
	 *
	 * @param threshold  The number of bytes at which to trigger an event.
	 * @param outputFile The file to which data is saved beyond the threshold.
	 * @param prefix     Prefix to use for the temporary file.
	 * @param suffix     Suffix to use for the temporary file.
	 * @param directory  Temporary file directory.
	 */
	private DeferredFileOutputStream(int threshold, File outputFile, String prefix, String suffix, File directory) {
		super(threshold);
		this.outputFile = outputFile;

		memoryOutputStream = new ByteArrayOutputStream();
		currentOutputStream = memoryOutputStream;
		this.prefix = prefix;
		this.suffix = suffix;
		this.directory = directory;
	}


	// --------------------------------------- ThresholdingOutputStream methods

	/**
	 * Returns the current output stream. This may be memory based or disk
	 * based, depending on the current state with respect to the threshold.
	 *
	 * @return The underlying output stream.
	 * @throws IOException if an error occurs.
	 */
	@Override
	protected OutputStream getStream() throws IOException {
		return currentOutputStream;
	}

	/**
	 * Switches the underlying output stream from a memory based stream to one
	 * that is backed by disk. This is the point at which we realise that too
	 * much data is being written to keep in memory, so we elect to switch to
	 * disk-based storage.
	 *
	 * @throws IOException if an error occurs.
	 */
	@Override
	protected void thresholdReached() throws IOException {
		if (prefix != null) {
			outputFile = File.createTempFile(prefix, suffix, directory);
		}
		FileOutputStream fos = new FileOutputStream(outputFile);
		memoryOutputStream.writeTo(fos);
		currentOutputStream = fos;
		memoryOutputStream = null;
	}


	// --------------------------------------------------------- Public methods

	/**
	 * Determines whether or not the data for this output stream has been
	 * retained in memory.
	 *
	 * @return {@code true} if the data is available in memory;
	 * {@code false} otherwise.
	 */
	public boolean isInMemory() {
		return !isThresholdExceeded();
	}

	/**
	 * Returns the data for this output stream as an array of bytes, assuming
	 * that the data has been retained in memory. If the data was written to
	 * disk, this method returns {@code null}.
	 *
	 * @return The data for this output stream, or {@code null} if no such
	 * data is available.
	 */
	public byte[] getData() {
		if (memoryOutputStream != null) {
			return memoryOutputStream.toByteArray();
		}
		return null;
	}

	/**
	 * Returns either the output file specified in the constructor or
	 * the temporary file created or null.
	 * <p>
	 * If the constructor specifying the file is used then it returns that
	 * same output file, even when threshold has not been reached.
	 * <p>
	 * If constructor specifying a temporary file prefix/suffix is used
	 * then the temporary file created once the threshold is reached is returned
	 * If the threshold was not reached then {@code null} is returned.
	 *
	 * @return The file for this output stream, or {@code null} if no such
	 * file exists.
	 */
	public File getFile() {
		return outputFile;
	}

	/**
	 * Closes underlying output stream, and mark this as closed
	 *
	 * @throws IOException if an error occurs.
	 */
	@Override
	public void close() throws IOException {
		super.close();
	}
}
